The invention is directed generally to color television receivers and, more particularly, to an automatic clock phase selector for use in such receivers.
Modern color television receivers commonly use the leading edge of a flyback pulse as a reference for generating burst-gates and automatic gain control (AGC) gates. Traditionally, the burst-gates and the AGC gates have been generated from the flyback pulse by analog circuitry. However, analog circuitry has timing problems due to inherent temperature, voltage, and circuit parameter variations in the system. Also, analog circuitry is expensive due to the required use of external components such as resistors, capacitors, and comparators.
Digital circuitry does not have the timing problems associated with analog circuitry. In addition, the use of external components is minimized. Consequently, digital circuitry has replaced analog circuitry for many television receiver operations. However, digital circuitry has not yet been utilized for generating timing period information to enable burst-gate and AGC gates due to its own inherent drawbacks.
Integrated-injection logic (I.sup.2 L) and transistor-transistor logic (TTL) are two types of digital logic which have been experimented with to provide a more efficient and less expensive circuitry for generating a burst-gate and an AGC gate from a reference flyback pulse. I.sup.2 L is the preferred digital logic because only two transistors are required per gate. Consequently, cost and space savings are possible with I.sup.2 L technology. However, the timing variation for the burst-gate and the AGC gate with reference to a leading edge of the flyback pulse must be less than 1/2 microsecond. In order to attain a timing variation of less than 1/2 microsecond, a 2 MHz clock input is required. Unfortunately, I.sup.2 L technology has a frequency response which, in its commercial applications, is limited to slightly under 2 MHz.
TTL technology easily meets the timing requirements of less than 1/2 microsecond variation because its frequency response may be as high as 70-80 MHz. However, TTL technology takes up too much space and is more expensive than I.sup.2 L technology because at least five to six transistors are required per gate. Furthermore, TTL technology cannot be processed on the same chip with linear circuitry.
Therefore, previous attempts to replace analog circuitry with digital circuitry to generate a burst-gate and an AGC gate from a reference flyback pulse have not generally been successful. The frequency response limitations of I.sup.2 L technology and the cost and incompatibility of TTL technology with linear circuitry have been the major obstacles barring the application of digital circuitry. The present invention overcomes the problems noted above with digital logic by providing a digital automatic clock phase selector for generating a burst-gate and an AGC gate from a reference flyback pulse.